1. Field of the Invention
This invention relates to gas detection and monitoring. More specifically, the invention is an optical-based system and method for detecting and monitoring the presence of gas in vacuum environments such as space.
2. Description of the Related Art
In terms of near-term, long-distance space travel, missions to the Moon and Mars are likely possibilities. Lunar and longer-term Martian missions may require the capability to store cryogenic liquids in an unmanned state for long periods of time. Storage containers filled with cryogenic fluids, helium pressurant, or methane propellant, could be placed on the lunar surface in advance of a manned landing. Earth Departure Stages (EDS) for trips to the moon and Mars could be “parked” in orbit for several months with the tanks thereof being maintained in a filled state. A method for determining whether these pressurized systems are in a ‘safe’ (i.e., non-leaking) condition is needed before resources are allocated for a rendezvous with a space crew. Furthermore, during the course of a mission, it becomes critical to monitor the system's health to ensure that no leaks develop and/or discover them early after their occurrence so corrective measures can be taken before the mission is endangered.
Since space environments are essentially vacuum environments, any leak detection and/or monitoring system/method must be capable of operating in a vacuum environment. In general, there are several methods to detect the presence of a gas in vacuum, but no off-the-shelf instrument is particularly well-suited as a candidate for leak detection that can occur on various times scales and lead to pressure levels in the vicinity of the leak that span several orders of magnitude. High-vacuum gauges (ion gauges) are quite accurate at low pressures, but they are relatively fragile and their filaments can become damaged if operation is attempted above 1 milliTorr for any prolonged period of time. Furthermore, high-vacuum gauges generate heat that could ignite a leaking combustible propellant. Mass spectrometers can be large and are difficult to locate in space-limited or remote locations. In addition, their operation requires high-voltage, the presence of which is generally not desirable next to a liquid hydrogen or oxygen tank. Reactive coupons (e.g., palladium-catalyzed silicon carbide) are generally species specific and can severely outgas in a vacuum environment to the point of uselessness. Several techniques to measure the amount of liquid in a cryogenic tank have been attempted or proposed, but none have been completely successful and most have fairly poor resolution. The sensitivity of these techniques does not allow detection of small leaks, or identification of the leak location.